Diluted magnetic semiconductors have received much attention due to their potential applications for spintronics devices. A prototypical system (Ga,Mn)As has been widely studied since the 1990s. The simultaneous spin and charge doping via hetero-valent (Ga3+,Mn2+) substitution, however, resulted in severely limited solubility without availability of bulk specimens. Here we report the synthesis of a new diluted magnetic semiconductor (Ba1−xKx)(Zn1−yMny)2As2, which is isostructural to the 122 iron-based superconductors with the tetragonal ThCr2Si2 (122) structure. Holes are doped via (Ba2+, K1+) replacements, while spins via isovalent (Zn2+,Mn2+) substitutions. Bulk samples with x=0.1−0.3 and y=0.05−0.15 exhibit ferromagnetic order with TC up to 180 K, which is comparable to the highest TC for (Ga,Mn)As and significantly enhanced from TC up to 50 K of the ‘111’-based Li(Zn,Mn)As. Moreover, ferromagnetic (Ba,K)(Zn,Mn)2As2 shares the same 122 crystal structure with semiconducting BaZn2As2, antiferromagnetic BaMn2As2 and superconducting (Ba,K)Fe2As2, which makes them promising for the development of multilayer functional devices.
Diluted Magnetic Semiconductors (DMSs) have received much attention due to their potential applications for spin-sensitive electronics (spintronics)1,2,3,4,5. DMS systems are produced by doping semiconductors with magnetic metal elements. In typical systems based on III–V semiconductors, such as (Ga,Mn)As, (In,Mn)As and (Ga,Mn)N, substitution of divalent Mn atoms into trivalent Ga (or In) sites leads to severely limited chemical solubility, resulting in metastable specimens only available as epitaxial thin films. The hetero-valent substitution, which simultaneously dopes both hole carriers and magnetic atoms, makes it difficult to individually control charge and spin concentrations for better tuning of quantum freedom.
Following a theoretical proposal by Masek et al.6, a new system Li(Zn,Mn)As was recently synthesized by Deng et al.7 based on the I–II–V semiconductor LiZnAs, showing a Curie temperature up to TC=50 K. In this system, charges are doped via off-stoichiometry of Li concentrations, while spins by the isovalent (Zn2+,Mn2+) substitutions. Although Li(Zn,Mn)As was a ferromagnetic DMS of a new type having a few distinct advantages over (Ga,Mn)As, the upper limit of currently achievable TC has been significantly lower than that in (Ga,Mn)As2,7.
BaZn2As28 is a semiconductor synthetized at high temperature (>900 °C) with the tetragonal ThCr2Si2 crystal structure (shown in Fig. 1a), identical to that of BaFe2As2 (9), BaMn2As2 (10,11) and (Ba,K)Mn2As2 (12). (Ba,K)Fe2As2 (9,13) is a classic member of the ‘122’ type iron pnictide superconductors with transition temperature up to 38 K, whereas BaMn2As2 is an antiferromagnet with TN~625 K (refs 11,12). It is noted that the stable phase at room temperature of BaZn2As2 crystallizes into a different orthorhombic structure with space group Pnma8 (Supplementary Fig. S1). However, we found that 10% of K or Mn doping dramatically stabilizes the tetragonal ThCr2Si2 structure at room temperature down to T=3.5 K (Supplementary Fig. S2). Here we report the synthesis of a new ferromagnetic DMS (Ba,K)(Zn,Mn)2As2 system, which shares the same ‘122’ structure with (Ba,K)Fe2As2 and (Ba,K)Mn2As2. Via (Ba,K) substitution to dope hole carriers and (Zn,Mn) substitution to supply magnetic moments, samples with 5–15% Mn doping exhibit ferromagnetic order with TC up to 180 K.
Synthesis and structural characterization
Polycrystalline specimens of (Ba,K)(Zn,Mn)2As2 were synthesized using the arc-melting solid-state reaction method similar to that described in ref.7. Details of the synthesis and instruments used for characterization are described in the Methods section. Figure 1b shows the X-ray diffraction results of (Ba1−xKx)(Zn0.9Mn0.1)2As2 for x=0.05, 0.1, 0.15, 0.2, 0.25 and 0.3, respectively. Patterns with a 2θ range (10°–80°) were collected, and the least-squares method was used to determine the lattice parameters of all polycrystalline samples, as shown in Fig. 1c. Compared with the lattice parameters a=4.121 Å and c=13.575 Å of BaZn2As2, the a axis expanded and the c axis shrank in (Ba1−xKx)Zn2As2. For compounds with Mn 10%, (Ba1−xKx)(Zn0.9Mn0.1)2As2, the c axis has similar tendency with Mn free (Ba1−xKx)Zn2As2, while the a axis does not show obvious variation with doping. These results indicate successful solid solutions of K and Mn.
Further structural studies have been made by neutron diffraction measurements from a powder specimen of (Ba1−xKx)(Zn0.9Mn0.1)2As2 with x~0.3 at the FRM-II reactor in TU Munich. The powder diffraction pattern, shown in Fig. 1d, fitted well to the structure found by X-rays, and the contrasting neutron scattering length of Mn and Zn allowed us to confirm random substitution of Mn atoms in Zn sites. The residuals of the Rietveld analysis were about 4–5%. There was no structural phase transition below T~300 K, which is in contrast to the case of antiferromagnetic (Ba,K)Fe2As2 (9). Because of the small average moment size (average 0.1 Bohr magneton per Zn/Mn site) and limited neutron intensity, we could not detect a conclusive signal from ferromagnetic spins below TC.
Resistivity measurements shown in Fig. 1e indicate that BaZn2As2 is a semiconductor. Doping K atoms into Ba sites introduces hole carriers, leading to metallic behaviour in (Ba,K)Zn2As2. The resistivity curves of (Ba1−xKx)(Zn0.9Mn0.1)2As2, for selected values of x up to 0.3, exhibit a small increase at low temperatures due presumably to spin scattering of carriers caused by Mn dopants. This variation of resistivity, similar to that of (Ga,Mn)N (ref. 14), is often observed in heavily doped semiconductors. Strictly metallic behaviour (with monotonic decrease of resistivity with decreasing temperatures) is not a precondition of having a ferromagnetic coupling between Mn moments mediated by RKKY interaction, as discussed in refs 1 and 15.
Magnetization, Hall effect and hysteresis
Figure 2a shows temperature dependence of magnetization in zero-field-cooling (ZFC) and field-cooling (FC) procedures under 500 Oe for (Ba1−xKx)(Zn0.9Mn0.1)2As2 specimens with x=0.05, 0.1, 0.15, 0.2, 0.25 and 0.3, respectively. Clear signatures of ferromagnetic order are seen in the curves, with corresponding critical temperature TC=5 K, 40 K, 90 K, 135 K, 170 K and 180 K, respectively. Above TC, the samples are paramagnetic and the susceptibility χ(T) can be fitted to the Curie–Weiss formula with an effective paramagnetic moment ~5 μB per Mn2+. The hysteresis curves M(H) at T=2 K in Fig. 2b exhibit an initial increase from H=0 state achieved by ZFC procedure, and a small H-linear component, which is presumably due to remaining paramagnetic spins and/or field-induced polarization. By subtracting this small T-linear component, we obtain the M(H) curves of (Ba1−xKx)(Zn0.9Mn0.1)2As2 at T=2 K shown in Fig. 2c. The saturation moment of 1~2 μB per Mn atom is comparable with that of (Ga,Mn)As1 and Li(Zn,Mn)As7.
The Hall effect measurements indicate that 10% (Ba,K) substitution in (Ba,K)Zn2As2 results in hole concentration of 4.3 × 1020 cm−3, consistent within a factor of two with that obtained by assuming that each K atom introduces one hole to the system. In (Ba,K)(Zn,Mn)2As2, linear dependence of Hall resistivity with magnetic field is observed above TC. As shown in Fig. 2d for the x=0.15 and y=0.10 system, having TC=90 K, the Hall resistivity deviates from the linear dependence in low field at TC. In the ferromagnetic state below TC, the anomalous Hall effect is observed with a small coercive field ∼35 Oe in the temperature region between TC and the history-dependence temperature THist below which FC and ZFC susceptibility shows deviation. The small coercive field above THist will be helpful for spin manipulation.
The deviation between the magnetization in FC and ZFC procedures in Fig. 2a and a large difference between the initial and the field-trained M(H) curves in Fig. 2b can be ascribed to a large coercive force demonstrated in Fig. 2c below THist. A very similar departure of ZFC and FC magnetization, associated with a coercive field~1 T, was observed in a well-known itinerant ferromagnet SrRuO3 (16,17), which has a comparable TC~160 K. The large coercive field at low temperatures in (Ba,K)(Zn,Mn)2As2 is different from Li(Zn,Mn)As, which exhibits a small coercive field of~100 G in the entire temperature region7. The large temperature dependence of the coercive field in (Ba,K)(Zn,Mn)2As2 opens a possibility of temperature tuning of anisotropy and stability of magnetic memory.
Muon spin relaxation (MuSR)
Using bulk polycrystalline specimens, we also performed positive MuSR measurements at Paul Scherrer Institute. Figure 3a shows the time spectra of the zero-field (ZF) MuSR on a (Ba0.8K0.2)(Zn0.9Mn0.1)2As2 specimen, which has TC~140 K, as determined by magnetization (inset of Fig. 3b). A sharp increase of the MuSR relaxation rate is seen with decreasing temperature below TC, and a highly-damped precession signal was observed below T~40 K. The volume fraction of magnetically ordered state was estimated by using MuSR data in ZF and weak transverse field of 50 G as shown in Fig. 3b. The MuSR results indicate static magnetic order developing in the entire volume with a rather sharp onset below TC.
The ZF precession spectra in Fig. 3a at T=5 K and 20 K look very similar to those observed in the ferromagnetic SrRuO3 by ZF-MuSR (see Fig. 7 of refs 18 and 19). Small oscillation amplitudes in both systems may be due to domain structures and spread of demagnetizing field in polycrystalline specimens. We also note that ZF-MuSR spectra did not show oscillation in ferromagnetic Li(Zn,Mn)As7 and (Ga,Mn)As15. In all these DMS systems, random substitution of Mn at Zn or Ga sites generates a spatially random distribution of Mn moments, which makes the local field at the muon site highly random even in the ferromagnetic ground state. Because of this feature, ZF precession signals are strongly damped.
In Table 1, we summarize transition temperature TC determined by ZFC and FC magnetization, and the size of the ordered moment per Mn at T=2 K, obtained from the H=0 values of the M(H) curve after cycling the field to 7 T, for (Ba1−xKx)(Zn1−yMny)2As2 with the K concentration x up to 0.3 and Mn concentration y up to 0.15. The highest TC is obtained for x=0.3 and y=0.1. We notice a tendency for the reduction of the moment size with increasing Mn doping, which may result from competition between antiferromagnetic coupling of Mn moments in the nearest neighbour Zn sites and ferromagnetic coupling between Mn moments in more distant locations mediated by the doped hole carriers. This feature is common to (Ga,Mn)As and Li(Zn,Mn)As.
Table 2 compares a few selected basic properties of the present compound with those of (Ga,Mn)As and Li(Zn,Mn)As. The present system is different from Li(Zn,Mn)As in several aspects: first, the currently available highest value of TC is more than three times higher in (Ba,K)(Zn,Mn)2As2 than that in Li(Zn,Mn)As. Furthermore, the present system shows a very high coercive field at low temperatures. In addition, notable differences lie in crystal structures. The crystal structure of the ‘111’ DMS system Li(Zn,Mn)As, is different from that of the relevant antiferromagnet LiMnAs and also from superconducting LiFeAs, although they share common square-lattice As layers. In the present ‘122’ DMS ferromagnet (Ba,K)(Zn,Mn)2As2, semiconducting BaZn2As2, antiferromagnetic BaMn2As2 and superconducting (Ba,K)Fe2As2 all share the same crystal structure shown in Fig. 1a, except for a small orthorhombic distortion in (Ba,K)Fe2As2 associated with antiferromagnetic order. Moreover, the lattice constants in the a-b plane match within about 5% as shown in Table 3. These features could provide distinct advantages to the present system over the 111 DMS systems in attempts to generate functional devices based on junctions of various combinations of these states.
Possible research use of such junctions includes, for example, quantitative estimation of the carrier spin polarization and scattering strength via Andreev reflection at the DMS–superconductor interface20. In conclusion, we presented successful synthesis of a new ferromagnetic DMS system with TC up to 180 K developed over the entire volume. Availability of bulk specimens, independent spin and charge controls and perfect lattice matching with the 122 FeAs superconductors and relevant antiferromagnets make this promising system decisively different from the existing DMS systems based on the III–V semiconductors.
Polycrystalline specimens of (Ba,K)(Zn,Mn)2As2 were synthesized using the arc-melting solid-state reaction method similar to that described in ref. 7. The starting materials, namely, precursors of BaAs, KAs, ZnAs, and high-purity Mn powders, were mixed according to the nominal composition of (Ba,K)(Zn,Mn)2As2. The mixture was sealed inside an evacuated tantalum tube that is, in turn, sealed inside an evacuated quartz tube. The mixture was heated until 750 °C at 3 °C min−1. Then, the temperature was maintained for 20 h before it was slowly decreased to room temperature at a rate of 2 °C min−1. Samples were characterized by X-ray powder diffraction with a Philips X'pert diffractometer using Cu K-edge radiation. The DC magnetic susceptibility was characterized using a superconducting quantum interference device magnetometer (Quantum Design, Inc.), whereas the electronic-transport were measured using a physical property-measuring system. Neutron powder diffraction measurements were performed at the FRM-II reactor in TU Munich using the SPODI spectrometer (for details of SPODI, see http://www.sciencedirect.com/science/article/pii/S0168900211021383.)
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The present work was supported by the Chinese NSF and Ministry of Science and Technology (MOST) through research projects at IOP; the National Basic Research Program of China (973 Program) under grant no.2011CBA00103 at IOP and Zhejiang; the US NSF PIRE (Partnership for International Research and Education: OISE-0968226) and DMR- 1105961 projects at Columbia; the JAEA Reimei project at IOP, Columbia, PSI, McMaster and TU Munich; and NSERC and CIFAR at McMaster. We would like to thank I. Mirebeau, S.Maekawa and L. Yu for helpful discussions.
The authors declare no competing financial interests.
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